Short-circuit detector



Fatented Dec. 31, 1929 UNITED STATES PATENT OFFICE CHARLES A. BUTCHER, OF FOR-EST HILLS, PENNSYLVANIA, ASSIGNOB TO WESTING- HOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYL- VANIA SHORT-CIRCUIT DETECTOR Application filed May 17, 1928. Serial No. 278,395.

This invention relates to protective s tems and, more particularly, to protective systems for direct-current distribution circuits such as are employed to supply energy to railway systems.

In my copending application, Serial No. SL858, tiled on January 16, 1926, I have disclosed a protective system involving the use of a direct-current relay which is energized in accordance with the rate, duration and direction of changes of current in a directcurrent transmission system.

The copending application of Roy J. WVeusley, Serial No. 672,829, filed November 5, 1923, and assigned to \Vestinghouse Electric & Manutacturing Company, discloses a pro tective system which also operates in response to the rate and direction of changes of current in a direct-current distribution system. The principal object 01" this invention is to improve upon the systems disclosed in the above mentioneo. applications and to provide a protective system for direct-(ailment distribution systems which will operate to isolate faulty sections of such systems upon the occurrence of faulty conditions therein.

Another object of this invention is to provide a protective system which will be un affected by the occurrence of legitimate overloads.

A further object of this invention is to provide a protective system capable of discriminating between short circuits and legitimate overloads, even though the latter be of higher absolute value than the former, and

regardless of whether the rate of current increase is greater or Another object of this invention is to 3 "oride a protective system which will operate promp to isolate a faulty portion ot a distribution system without interrupting service on the ren lining portions thereof.

A still further object of this invention is ,5 to provide a protective system combining the quick response of a solenoid-and-plunger type relay for isolating a circuit section on the occurrence of a nearby fault, and the in herent, inverse time characteristic and the ballistic effect-of a galvanometer or DArsonthe circuit-breaker.

Further objects and advantages of the system o1 my invention will become apparent as the description thereof proceeds.

For a complete understanding or" this invention, reference is made to the accompanying drawings, in which,

Figure 1 is a diagram illustrating the circuits and apparatus employed in one modification of my invention.

Fig. 2 is a circuit diagram of the usual direct-current distribution circuit in connection with which the system of my invention tinds its principal field of utility.

Fig. 3 illustrates graphically the variations with time of the short-circuit current in the distribution system and the current in the relay of the protective system of my invention; for the fault condition shown in Fig.

Fig. 4r is similar to Fig. 3, but illustrates a .dil'ierent condition of the distribution system; and

Fig. 5 illustrates another modification of my invention which is especially do; loned to aiford protection against the conditions illustrated in Fig. 4..

Referring particularly to Fig. l, a directcurrent distribution circuit 10 is adapted to be connected, by a circuit breaker l1 .to any suitable source of supply (not shown). The circuit breaker 11 is adapted to be closed by the energization of its closing coil 12 w-hicl may be connected to an auxiliary source of current 13 by the closing of switch 14, which may be controlled automatically as well as manually. The circuit-breaker 11 is pro- I cooperates.

vided with a latching mechanism 11, which, when th circuit breaker is closed, maintains it in that position so that the closing coil 12 may be (lo-energized without causing th breaker to open. A tripping coil 16 is provided to positively unlatch the circuit breaker 11 upon the occr rence of predetermined con ditions, whereby the breaker is opened immediately by a spring 12, or other suitable means.

Connected in the circuit between the current source and the circuit breaker 11 is a series or impulse transformer 17. A tran sient voltage is induced in this impulse transformer whenever a value of the current in the feeder circuit changes, the direction of the voltage being dependent upon the direction of the change of current. For; controlling the circuit-breaker 11, I provide protective relays 20 and 21 having operating coils 18 and 19, respectively, connected in series to the impulse transformer 17. These operating coils are energized by the transient current resulting from the voltage induced in the impulse transformer 17 when the magnitude or direction of the current in the circuit 10 changes.

A variable inductance 18 is connected in series with the moving coil 18 of the relay 20 for a purpose which will appear hereinafter.

The relay 20 is a galvauometer relay of the DArsonval type and consists of a magnetic circuit 22 with which the operating coil 18 The magnetic circuit 22 may be either that of an clectromagnet or a permanent magnet and, as shown, is an electromagnet having its winding connected to the source 13. The moving coil 18 actuates a contact arm 23 which is adapted to bridge the contacts 2 1 to perform a function which will be described later.

The moving coil 18 of the relay 20 is wound upon an aluminum frame which constitutes an inductive damping means as well as a mechanical support for the coil.

It is well known characteristic of devices of the galvanometer type that the rotation of the moving coil is dependent upon both the magnitude and the duration of the energizing current. Thus, in the system of my invention, the relay 20 functions in response both to the average rate and to the duration of changes of the current in the distribution circuit.

In addition to the relay 20, I provide also the relay 21 which is a simple solenoid-andplunger relay having a moving contact 25 adapted to bridge the fixed contacts 26. The ope atihg coil 19 of the relay 21 is energized by the same current which traverses the moving coil 18 of the relay 20. The moving element 25 of the relay 21 is small and light in weight, and its operation, therefore, is almost instantaneous. The relay 21 funct-ions largely in response to the peak value of the energizing current or, in other words, in the system of my invention, substantially to the average rate of change of the current in the circuit 10. Besides the operating coil 19, the relay 21 is provided with a polarizing coil 27 which is continuously energized from the source 13. The function of the polarizing coil 27 is to prevent operation of the relay 21 upon the occurrence of a decrease in the current in the circuit 10, and this object is accomplished by so connecting the winding 27 that its magnetic effect aids that of the operating coil 19 when the current in the circuit 10 increases but opposes the magnetic effect of the coil 19 when the load currentdecreases.

An auxiliary relay 28 has its operating coil connected in a circuit including the contacts 2 1 of the relay 20 and contacts 26 of the relay 21, which are in parallel therewith, an auxiliary front contact 29, which isclosed when the circuit-breaker 11 is closed, and the current source 13. When the relay 28 is energized, its contact 30 is closed and eompletes a circuit from the source 13, through the contact 29 on the circuit-breaker, the contact 30 of the relay 28, the circuit-breaker trip coil and thence to the source 13. The operation of the relay 28 also completes a circuit in parallel with the contacts 24 and 26 so that its coil is maintained energized independently of the engagement of the contacts of relays 20 and 21 until the circuit is opened at contact 29 which opens when the tripping coil 16 is effective to cause the opening of the circuit-breaker 11.

The modification of my invention already described will. operate successfully to discriminate between short circuits and legitimate load currents even though the latter are greater than the former in absolute value and regardless of whether the rate of current increase is greater or less. This method of operation results from the fact that the system of my invention responds, not to the maximum value of current increments in the load circuit, but to both the average rate of change of current and to the duration of such change.

As already stated, itis the function of the relay 20 to respond to distant faults which cause the current in the load circuit to increase with a comparatively slowly-rising characteristic. This slowly-rising characteristic is a result of the relatively high resistance and inductance in the load circuit when the fault is located at a distance from the source. The relay 20, because of its ballistic characteristic, is actuated in response to a slowly-rising current increment which has a peak value lower than that required to effect the operation of the relay 21, through the energization of its operating coil 19, by the voltage induced in the impulse transformer 17.

The relay 21, on the other hand, serves to isolate severe short-circuits which are caused by faults occurring relatively close to the supply source. In this case, the resistance and inductance of the circuit are low, and the average rate of rise of shortcircuit current is much greater than that of any legitimate load increment. The relay 21 is designed to operate in response to such conditions and will actuate its contact almost instantaneously upon the occurrence of such a short-circuit.

The relay 20 also operates on near-by shortcircuits but only after a definite time delay inherent in the relay because of its inductive damping. The principal function of the relay 21 is to cause the tripping of the breaker 11 substantially instantaneously in installations in which the fault current is reduced or interrupted by the opening of a high-speed circuit-breaker in the feeder circuit.

The protective system of my invention, therefore, operates to guard against shortcircuit conditions at any point in the distribution system. False operation, as a result of load increments, is prevented by the current-time characteristic of such a circuit condition, in response to which the protective system of my invention does not operate.

As already stated, the relay 20 operates not only in response to the average rate of change of current in the circuit 10, but also 111 re sponse to the duration of such change.

Because of the fact that there is always a difference in the characteristics of short-circuits and normal lead increments, and because the system of my invention may be adjusted to operate under a wide range of Varying conditions, I am enabled to detect and isolate short-circuits, and, at the same time, avoid interruption of service on the load circuit upon the occurrence of a legitimate overload, even though the latter is much greater in absolute value than the former, whatever the constants or characteristics of the system may be. i

As a further means for preventing the relay 20 from responding to a sharply rising current increment, where such operation is desirable, I employ a non-inductive shunt 18 connected across the moving coil 18 of the relay 20. Although the inductance of the winding of the moving coil 18 is slight, it is large in comparison with that of the non inductive shunt 18. When a transient voltage is induced in the impulse transformer 17, as a result of a sharply rising increment in the load current, the impulse current in the relay circuit will divide between the resistor 18 and the coil 18 in inverse proportion to their impedances. Thus, only a portion of the impulse current passes through the coil 18, while the remainder passes through the resistor 18. This further reduction in the current through the operating coil. of the relay 20, upon the occurrence of'a high-rate increment in the load current, tends to prevent the relay from operating when the load current increases at a very high rate. When the current increase is of a slowly rising characteristic, however, the impulse current divides between the coil 18 and the resistor 18 substantially in inverse proportion to the resistances of these elements, and the coil receives a greater portion of the impulse current than in the case previously described, so that the relay 20 will operate to bridge its contact members 24 in response to a slowly-rising current increment but will be affected only slightly by a current increment of sharplyrising characteristic. This division of the current from the impulse coil 17 may be controlled by adjusting the variable inductance 18 in series with the moving coil "18 of the relay 20.

For a further explanation of the operation of my protective system, reference is made to Fig. 2 illustrating a direct-current dis tribution system in which my protective system is found useful.

In Fig. 2, sections of a trolley wire or third rail are indicated at 35, 36 and 87. These sections of trolley wire may be energized from suitable sources, such as the gen erators 88 and 39, through circuit-breakers 40, ll or a2, 43 which connect the generators 38 and 39 to sections 14, and 46 of a feeder cable. The feeder sections are connect-ed to the trolley at intervals by the taps 47. It is to be noted here, in passing, that a third rail conductor will have a considerably greater inductance than a copper trolley wire or feeder conductor.

Let it be assumed that a short circuit occurs at A. This fault will be fed from the two sources 88 and 39, through the feeder sec-' same final value as that to which it would have risen had the breaker 41 been feeding the short circuit A alone. The current through the breaker 41 thus rises to its final value in two steps which may be seen in Fig. 8, which illustrates graphically the conditions existing in various portions of the distribution circuit upon the occurrence of a fault, such as that at A.

In Fig. 3, the curve 50 illustrates how the current supplied by the source 38 to the fault A would increase if no current were supplied to the fault by the source 39. The curve 51 illustrates the impulse current which would be induced in the impulse transformer 17 by a current increment of the characteristic illustrated by the curve 50. When the fault A is ted from both sources 38 and 39, however, the current supplied by the source 38. through the breaker il, rises in the manner 'llustrated by the curve 52 in accordance with the foregoing explanation. The current increase takes place in two steps and, consequently, two separate impulses 53 and iii are induced in the transformer 17 It is apparent that any device which tunetions only in response to average rate of rise of feeder current or, in other vords, to the peak value of the impulse current, must be set to trip on the smaller impulses 58 and This setting, however, would be low enough to cause a relay, such as the relay 21, to be cp -rated by legitimate load increments having a peak value within the capacity of the generating equipment. In the system of my invention, however, the relay 21 may be set to respond only to impulse currents of the peak value of that shown by the curve 51, and the relay may be relied upon to operate when. the load current increases in accort ance with the curve 52. This relay, because of its ballistic characteristic, adds the eitects of the two impulses 53 and i and bridges its contact members 2a to cause the opening of the circuit-breaker.

For the ourposc of distinguishing between short-circuits and load increments of closely similar characteristics, I have devised a modiiication of my invention.

Referring to Fig. 4-, let it be assumed that a fault current has the characteristic shown by the curve 60, and that a legitimate load increment has a characteristic, such as that shown by the curve ill. Since the curves 60 and 6! coincide between the origin and the point (32, it is obvious that any means for discriminating between the two conditions must be set to operate at a time later than that of the point 62.

The means which I have provided for discrin'iinating between legitimate loads and short circuits of closely similar characteristics is shown in Fi 5, which, in general, is simi lar to the circuit of Fig. 1. The difference con. sts in that a compound. winding 71 is provided for the relay 20, being connected in series with a resistor 73 across a non-in ductive shunt T l. An inductance 72 connected in series with the shunt 7t and both are shunted by a resistor 7 In all other respects, the

circuit of Fig. 5 is identical with that of Fig. i and operates in .a similar manner to cause the tripping of the circuit-breaker 11 under appropriate conditions.

By the provision of the winding 71, the inductance 72, and the shunts 7 4 and 7 5, however, the operation of the relay 20 is made to depend, not only upon the rate and dura tion of changes in the load current, but also upon its magnitude. The inductance 72 provents the compound winding 71 from becoming fully energized until a time at which the characteristics of the short circuit and the legitimate load diverge, by absorbing the greater portion of the voltage drop across the shunt 7 5 so long as the load current rises at a relatively rapid rate. Because of the added flux induced in the magnetic circuit of the realy 20 by the compound winding 71, when it does become energised, a greater torque is exerted on the moving coil 18 by the occurrence of a fault having a characteris tic illustrated by the curve (30 than by the occurrence of an overload having a characteristic illustrated by the curve 61. The torque on the moving element of the relay is proportional to the area enclosed by the curves illustrating the current through the moving element under the two conditions. These curves are illustrated at and 81 in Fig. 4:.

The curve 80 illustrates the current passing through the moving coil 18 of the relay 20 when a shortcircuit having the characteristic illustrated by the curve (50 occurs on the circuit, while the curve 81 illustrates the current passing through the relay winding as a result of a load increment having the characteristic illustrated by the curve 61. The curve 80 obviously encloses a much greater area than the curve 81 and, therefore, the occurrence of a short-circuit exerts a much stronger turning impulse on the moving clement of the relay 20 than does the occurrence of a legitimate overloadof a similar rising characteristic. The distinguishing feature of the two conditions resides in the fact that, when a legitimate load is placed on the circuit, although the current increment may be of nearly the same characteristic as a shortcircuit, it continues to risefor a limited period only, and then starts to droop because of the eil'ect of the counter-electromotive force developed by the load devices, such as railway motors, whereas, under short-circut conditions, the current increases steadily to its final value, at a. rate dependent on the circuit constants.

It is apparatus from the curves in Fig. l that the turning in'ipulse on the moving coil of the relay 20 is considerably greater, on the short-circuit condition, after the time at at which the curves 60 and 61 diverge, than would be the case it there were no compounding of the field flux of the relay.

From the foregoing description, it will be apparent to those skilled in the art that the system of my invention provides complete protection for direct-current distribution circuits and that, While it causes prompt isolation of faulty portions of such circuits, itdoes not operate as a result of legitimate load increments, even though the absolute value of the latter is greater than that of the former.

I am aware that it has been previously proposed to make use of a galvanometer relay for direct-current distribution-system protection and that it has also been suggested to use a polarizing plunger type of relay, but I believe I am the first to combine these devices into a single protective system having the novel characteristics and advantages hereinabove pointed out.

Since it will be obvious to those skilled in the art that the system disclosed herein may be changed in many respects, I do not intend my invention to be limited to the specific modifications disclosed, except as necessitated by the appended claims.

I claim as my invention:

1. In a protective system for a directcurrent distribution system, a circuit-inter rupter for disconnecting a faulty section from a supply source, means for controlling the operation of said interrupt-er including a re lay operative in accordance with the average rate of change of current in the system and a relay responsive to both the average rate and duration of such change.

2. The combination with a direct-current distribution system, a circuit-interrupter therein for isolating portions thereof, of means for controlling the operation of the circuit-interrupter including a relay responsive to the average rate of current rise in said system and a relay operative in response to the magnitude of the current and the rate and duration of such increase.

3. 111a protective systeinfor a direct-current distribution circuit, a circuit-interrupter in said circuit and means for controlling said interrupter including a solenoid relay in stantaneously responsive to the average rate of change in the current in said circuit, and a ballistic relay responsive, after a time delay, to the average rate, the duration and the direction of such change.

at. In a protective system for a direct-cub rent circuit, a circuit-interrupter therein and means for controlling said interrupter including a relay responsive instantaneously to brief, high-rate current changes in said circuit and a relay responsive after a time delay to extended, low-rate changes, whereby said interrupter is opened on the occurrence of short-circuits but not on legitimate overloads, regardless of the relative absolute values of the current under such conditions.

5. A relay system for protecting a circuit, capable of discriminating between legitimate overloads and short-circuits, comprising a circuit-interrupter in said circuit, arelay responsive to the average rate of rise of the current in said circuit and a relay responsive to the average rate, the duration and the magnitude of such rise, and means responsive to the operation of either of said relays to cause the opening of said interrupter.

6. A protective system for a direct-current circuit comprising a circuit-interrupter and an impulse transformer connected in said circuit, relays having windings connected across-said transformer, one of said relays being responsive instantaneously and another being responsive after a time delay, and

means controlled by said relays to cause the opening of the interrupter.

7. A protective system for a direct-current circuit comprising a circuit-interrupter and an impulse transformer connected in said circuit, an instantaneously responsive solenoid relay and a galvanometer relay of the DArsonval type responsive after a time delay, said relays having their windings connected across said impulse transformer, and means controlled by said relays for causing the opening of said interrupter.

8. A protective system for a direct-current circuit comprising a circuit interrupter and an impulse transformer connected in said circuit,.an instantaneously responsive solenoid relay and an inductively damped galvanon'ieter relay having a moving coil connected with the coil of said solenoid relay across the impulse transformer and a fixed coil adapted to be connected to a substantially constantvoltage source of current, and means controlled by said relays for opening said interrupter.

9. A protective system for a direct-current circuit comprising a circuit interrupter and an impulse transformer connected in said circuit, a plunger type relay having a biasing coil and an operating coil and a ballistic galvanometer relay having a coil connected with said operating coil across said impulse transformer, said biasing coil being adapted to be connected to a constant-voltage source so that its magnetic efiect aids that of the operating coil when the latter is energized by an increase in the current in said circuit, and neutralizes the effect of the operating coil when the latter is energized by a decrease in the current in said circuit, and means controlled by said relays for opening the interrupter.

10. A protectivesystem for a direct-current distribution circuit comprising a circuit-interrupter and an impulse transformer connected in said circuit, a plunger relay having an operating coil, a moving contact and a biasing coil, and an inductively damped ballistic galvanometer relay having an operating coil, a moving contact and a polarizing coil, the operating coils of said relays being connected in series across said impulse transformer, the biasing and polarizing coils being adapted to be energized so that upon an increase in the current in the circuit, the relay contacts are actuated in one direction, and upon a decrease in said current. the contact of said first mentioned relay is unaffected and the contact of the galvanometer relay moves in a reverse direction, and means controlled by said relays for opening the interrupter.

11. In a protective system for a directcurrent circuit, the combination with a circuit-interrupter and an impulse transformer connected in said circuit, of means for con.- trolling said interrupter comprising a relay oi the DArsonval galvanometer type having a fixed coil adapted to be energized constantly and a movingcoil connected to said impulse transformer, a moving contact actuated by said movingcoil and a non-inductive shunt in parallel with said moving coil, and means controlled by said relay for opening said interrupter.

12. The combination, in a protective sys tem, with a distribution circuit, a shunt, an impulse transformer and a circuit-inter.- rupter therein, of a ballistic relay of the DArsonval ga-lvanometer type having a magnetic circuit, and a moving coil cooperating therewith connected to said impulse transformer, a non-inductive shunt connected across said coil, means for exciting said magnetc circuit comprising a coil connected across said shunt, and coil adapted to be constant.- ly energized, and means controlled by said relay for opening said interrupter.

13. The combination, in. a protective system, With a distribution circuit, an inductance, a shunt, a series transformer and a circuit-interrupter in said circuit, of an inductively-damped galvanometer relay having fixed and moving coils, the moving coil being connected to said transformer and in parallel with a nonrinductive resistor, one of said fixed coils being adapted to be connected to a constant-voltage current source, and another of said fixed coils being connected across said shunt.

14-. The combination with a direct-current distribution system, a ircuit-interrupter therein for isolating portions thereof, of means for controlling the operation of the circuit-interruptor including means responsive to the average rate of rise of current in said system and means operative in response to the magnitude of the current and the rate and duration of such increase.

In testimony whereof, I have hereunto subscribed my name-this 12th day of May,

CHARLES A. BUTCHER. 

